Storage units for electric energy are usually constructed from individual storage elements. The nominal operating voltage of these storage elements is usually at relatively low voltage levels, e.g. in the range of 2 to 2.5 V with Ultracap capacitors. For most applications, however, a far higher operating voltage is required for the energy storage unit. For example, the electric drives of hybrid motor vehicles, depending on the type, are operated at 48 V to 300 V, so that an energy storage unit used to feed a drive of this type needs to have a correspondingly high operating voltage. In order to achieve this high voltage level, with energy storage units, a corresponding number of storage elements is typically connected in series. For example, a 48 V energy storage unit can be formed by a series circuit of 20 Ultracap capacitors with a nominal operating voltage of approx. 2.5 V.
In the interim, it has become known that the service life of energy storage units of this type, which are constructed from a series circuit of a relatively large number of individual storage elements, is significantly shortened due to inhomogeneities in the charge condition of the individual storage elements (as cited for example in H. Schmidt et al., “The charge equalizer—a new system to extend battery lifetime in photovoltaic systems, U.P.S. and electric vehicles”, International Telecommunications Energy Conference, Intelec, Paris, 27 to 30 Sep. 1993, IEEE, vol. 3, Cord 115, pp. 146-151). Previously, it had been assumed that with a series circuit of individual storage elements, all of these elements had identical properties, and were always in the same charge condition. In fact, however, each storage element has individual parameters (such as capacity, self-discharging rate), which generally deviate slightly from those of the other elements. With a simple series circuit, differences of this kind can on the one hand—with discharging procedures—lead to low discharging or even inverse charging of storage elements with a low level capacity, while on the other hand—with charging procedures—leading to overcharging of storage elements which have been fully charged too early. This behaviour is generally divergent, i.e. even small differences between the individual storage elements lead in the course of time to the events listed above, when only a sufficiently high number of charging/discharging cycles is completed. The events described above initially lead to damage or failure of the affected storage element, and can finally—as a type of chain reaction—lead to the premature failure of the entire energy storage unit.
In order to avoid effects of this nature from differences (which are unavoidable in practise) between individual storage elements, several authors have already suggested different methods with which an enforced symmeterisation of the charging condition of the individual storage elements is conducted, such as Schmidt et al. in the conference contribution mentioned above and in EP 0 432 639 A2, N. Kutkut et al. in “Dynamic equalization techniques for series battery stacks”, Telecommunications Energy Conference 1996 (Intelec), Boston, 6 to 10 Oct. 1996, IEEE 0-7803-3507-4196, pp. 514-521, and Ridder in EP 1 283 580 A2. These suggestions share the basic concept of monitoring the voltage in the storage elements, and to remove a charge from higher charged storage elements and/or to feed a charge to storage elements which have a lower charge (whereby with some of the suggestions, a charge is removed from or fed to all storage elements, although more is removed from the higher charged elements, or less is fed to them than to the lower charged elements). According to Schmidt, a method is also known of forming sub-groups with storage elements which are connected in series, and of monitoring and symmeterising these as a unit (instead of all storage elements individually) in the manner described. While with earlier suggestions (which are reported by Schmidt, for example, in the conference paper mentioned above), the energy removed from the higher charge storage elements was dissipated in heat resistances, according to more recent suggestions, the removed energy is fed back to the energy storage unit (i.e. in effect, to the other storage elements), or alternatively, the energy fed to the lower charged storage elements is removed from the energy storage unit (i.e. in effect, from the other storage elements). A re-storage of this nature is more advantageous in terms of the efficiency, since in this way, it is not the entire re-stored energy which is lost, but only the losses entailed by the re-storing procedure. With these more recent suggestions, the service life of energy storage units of the type described above can be significantly extended—while maintaining a relatively high degree of efficiency.
As far as can be seen, however, the above mentioned publications provide no information as to how the charge redistribution circuit which is used for symmeterisation can itself be supplied with current. Presumably, the authors of the documents mentioned above intended the charge redistribution circuit to be supplied with the current required for its operation either from an external current source, or from the energy storage unit to be symmeterised itself. Elsewhere, a suggestion has already been made in another context of equipping individual battery cells with a control device which obtains its operating current from the battery cell to be monitored (U.S. Pat. No. 6,163,131). With a battery diagnosis device with measuring modules for one individual battery cell respectively, or—alternatively—for one group of battery cells respectively, it has also been suggested that this measuring module draw its operating current in each case from the monitored cell, or the monitored group of cells (DE 199 21 675 A1).
Overall, with a charge redistribution circuit which selectively removes or feeds a charge from or to individual storage elements or groups of storage elements (e.g. according to FIG. 5 of the above mentioned conference contribution made by Schmidt), a functioning voltage symmeterisation can be conducted. However, this circuit offered by Schmidt, for example, is rather expensive to produce. The object of the present invention is therefore, based on this prior art, for example, to provide a relatively simple charge redistribution circuit which enables a voltage symmeterisation of the storage elements of an energy storage unit.